Meeting the increasingly stringent nitrogen (N) and phosphorous (P) effluent standards has had a major impact on the design and operation of wastewater treatment facilities dealing with domestic sewage with their typical unfavorable characteristics. Since the first success in achieving biological P removal in a continuous full-scale biological N removal plant in the 1970's, incorporation of biological P removal in a biological N removal plant is considered to be a generally achievable objective. Design and operation of biological nutrient removal (BNR) plants are now required to optimize these two parallel but interactive processes to maximize both nitrogen and phosphorus removal. Design and operation also requires simultaneous control of the associated sludge bulking problems resulting from the proliferation of filamentous bacteria.
The available BNR processes can be divided into continuously and intermittently operated systems. Continuously operated systems comprise a number of separate tanks or ponds through which wastewater and sludge is passed in various ways. Intermittently operated systems use a single reactor or pond, sometimes separated into zones by baffling, with only one pass of the wastewater through the reactor pond. Intermittent processes can therefore be characterized by their unique repeated sequencing time-oriented operation as compared to the space oriented operation of the continuous processes.
Intermittently operated systems can be either fed continuously or intermittently. They can be also subdivided into variable and constant volume systems. The variable volume systems accomplish solid-liquid separation in the same tank with subsequent withdrawal of the treated effluent (intermittent decant) while the constant volume intermittently operated facilities carry that out by a separate in-series secondary clarifier or basin with or without an underflow recycle returning the activated sludge back to the process.
In the operation of intermittently fed sequencing batch reactors (SBR) or sequencing batch ponds (subsequently called reactors) a substantial proportion of the cycle time is used for the fill period. During this time, the part of the reactor volume that was discharged at the end of the previous cycle, is replaced by fresh sewage before aeration commences. In BNR operation of these reactors, the fill period is of major importance for the removal of both nitrogen and phosphorus based nutrients. There are strong indications that good nutrient removal performance is dependant on the structure and composition of the biomass flocs in the reactors. Flocs should ideally be of similar size, compact, spherical and without filamentous growth. This encourages simultaneous nitrification-denitrification during aeration periods and ensures good sludge settling properties. Several advantages of simultaneous nitrification-denitrification have been reported in the past including reduced requirements for biodegradable carbon (or COD) in the raw wastewater, reduced aeration requirements and part or complete elimination of anoxic reactors or sequences if net nitrate production can be kept at low levels. Achieving simultaneous nitrification-denitrification is therefore regarded as beneficial both in continuous and intermittent systems.
Existing technology such as the cyclic activated sludge system (CASS) uses so called selectors or contactors which are small volumes in the inflow part of the reactor. In these zones the inflowing feed is mixed with the return activated sludge which is pumped from the reactor bottom or from specific clarifiers. This has two major drawbacks. Firstly, only part of the sludge mass is contacted with the inflowing feed and secondly, it requires mechanical pumping of the sludge. This second requirement is not only operationally difficult but is likely to have a negative effect on the structure of the sludge flocs due to the mechanical stress exerted during the pumping action.